A traditional image is created when a sensor is placed in an optical system at a plane optically conjugate to an object which is to be imaged. This is the plane at which the best focus is achieved and therefore the best optical resolution of features in the object results. By design, rays emanating from a point within the object plane in multiple directions are captured by the optical system and those rays converge to approximately a single point in the conjugate image plane. The set of rays which are summed at any image point is generally constrained by physical apertures placed within the optical assembly. The traditional sensor records the summation of the intensity of light in the plane of the detector. The measurement contains the intensity distribution of light within the plane of the sensor but loses all information about the rays' direction before the summation. Therefore the process of recording a traditional image throws away a very large fraction of the information contained in the light absorbed.
As described by Ren Ng, the “light field” is a concept that includes both the position and direction of light propagating in space (see for example U.S. Pat. No. 7,936,392). The idea is familiar from the ray representation of light. We know that light energy is conserved as it passes along straight line paths through space. The light energy can be represented in a 4 dimensional space L(u,v,s,t) with an intensity value at each of (u,v) positions within a plane, and at angular rotations (s,t) about each of those axes. The concept is used extensively in computer graphics simulations. With the information from a light field, the rays can be propagated to destinations in other planes. The process of computing the light intensity at another plane and presenting it as if it were imaged on a virtual film is also called reconstruction. The methods described by U.S. Pat. No. 7,936,392 B2, as well as the doctoral thesis by the same author (R. Ng, “Digital light field photography” 2006) are exemplary descriptions of lightfield sensor technology, the mathematics for propagation of the light fields, and the practice of image reconstruction techniques using light fields, both of which are hereby incorporated by reference
A light field sensor for use in a digital focus camera is achieved by placing a sensor array 101 at or near the back focal plane of a lens array (lenticular array) 102 as illustrated in FIG. 1. This light field sensor is placed in a supporting assembly containing other optical components such as a main lens 103 to shape and constrain the light from the subject 104 to best fit the light field sensor geometry. In this way a ray is constrained in position by the individual lens in the array (lenslet) through which it passed, and in angle by the specific sensor pixel it is incident upon behind the lenticular array. Light field sensors may be created by other means known currently or by other methods which are likely to be devised in the future. One such alternative light field sensor may use an array of pinholes instead of a lenticular array. Another alternative may place the sensor array at a distance significantly different from the back focal plane of the lenticular array (Lumsdaine et al. “The Focused Plenoptic Camera”, ICCP April 2009). Such variations may achieve advantages in terms angular or spatial resolution given a particular sensor or lenticular array spacing. Ren Ng describes properties of generalized light field sensors in his dissertation work which extend beyond the format of the simple lenticular array placed in front of a sensor array. It is to be understood that all such representations are included if we speak of a lens array as one such representation of a light field sensor.
The light field sensor concept has been exploited by Ren Ng, and others to create cameras which can digitally focus an image after it has been acquired. In a camera demonstrated by Ng et al, a 16 megapixel array was placed at approximately the rear focal plane of a 90,000 microlens array, such that each microlens should cover a sensor area of about 13×13 pixels. This light field sensor was then placed such that the microlens array was located at the back focal plane of a traditional camera such that the scene was approximately focused on the lens array. The resolution of reconstructed images was then 300 by 300 pixels. This example highlights the cost of the light field sensor—the resolution of the reconstructed image is much less than the number of pixels required in the sensor. However as pointed out in Ren Ng's thesis, the technological capacity to create image sensors with high resolution far exceeds traditional imaging demands. Canon announced in 2010 an image sensor with 120 megapixels for use in an SLR type camera. With consumer level commercialization efforts underway for light field cameras it is reasonable to assume that image sensor technology will evolve to meet demand for megapixel resolutions. A similar camera described by Lumsdain and Georgiev demonstrates how adjustments to the light field sensor geometry may be used to optimize resolution vs. depth of field. Raskar et al describe the problem of glare in photography in terms of light fields and demonstrate how glare behaves in ray space as a high frequency noise that can be reduced by outlier rejection (see for example Raskar et al. “Glare Aware Photography: 4D Ray sampling for reducing glare effects of camera lenses” Mistubishi Electric Research Laboratories 2008 and U.S. Pat. No. 7,780,364).
Traditionally imaging of the retina has been performed by a fundus camera as recently reviewed by DeHoog et al. (DeHoog et al., “Fundus camera systems: a comparative analysis,” Appl. Opt. 48, 221-228 (2009)). A schematic for a typical fundus camera is shown in FIG. 2. Light from illumination source 12 is directed along an illumination path 201 containing a series of lenses 11, 9, 7 and apertures 10,8, before being directed to the eye 1 of a patient using an annular minor 3 and objective lens 2. Light reflected from the eye 1 passes through the center of the annular minor 3 and into a collection arm where it is imaged at image plane 6 after passing through focusing lens 4 and photographic lens 5. The primary challenge in fundus imaging compared with other types of imaging is a large amount of unwanted light coming from structures other than the intended target of examination. Primarily this is the result of glare specularly reflected and scattered from surfaces in the imaging path. This is traditionally managed by careful design of the illumination and collection paths, including complex apertures at precisely aligned positions. Crossed polarization between illumination and collection systems has also been used to suppress corneal glare (see for example U.S. Pat. No. 4,998,818).
An additional challenge of fundus photography is that the operator or control system of the device must precisely align the camera to the patient's pupil each time a photograph is taken to avoid introducing glare artifacts to the image originating from light reflected and scattered from the cornea and iris of the eye. The need to precisely locate the camera to the eye has led to further complications in the design of the fundus camera. The most intuitive means to align the optical axis of a device to the pupil of the eye is to image the pupil directly along that optical axis and provide that image to the operator or control system. Because the fundus imaging path is typically designed to block any light scattered from the iris, and any optical element present in both the illumination path and the imaging path is a potential light scattering risk, Tawada describes a fundus camera which includes a separate imaging path for imaging the iris. This imaging path is removably coupled to the optical axis by a flip in mirror to avoid introducing artifacts to the fundus image during actual fundus imaging. Such a solution adds optical and mechanical complexity to a system design, resulting in additional design constraints, increased cost, and reliability risk. It is therefore an object of the present invention to apply a new sensor to fundus photography to overcome the above described limitations in the prior art.